Foamy viruses are a group of retroviruses that infect a wide variety of mammals including humans but have yet to be definitively associated with disease. Since the original isolation of the human foamy virus (HFV) in 1971, research on this retrovirus has been limited relative to the other human retroviruses especially in relation to its structural proteins. Nevertheless, HFV should be studied extensively because 1) its pathogenic potential has not been determined with certainty; 2) it displays numerous unique characteristics in comparison to other retroviruses; and 3) it has potential use a vector for gene or vaccine therapy in the future. HFV displays three unique characteristics that we propose to study more extensively: 1) like all foamy viruses it induces prominent vacuolization of infected cells and virus budding is directed primarily into these structures; 2) the Gag polyprotein of foamy viruses is transiently transported to the nucleus shortly after translation; and 3) we have discovered and recently reported an endoplasmic reticulum retrieval signal in the HFV glycoprotein cytoplasmic tail, a finding with major implications for the unique life cycle of HFV. These poorly understood features are novel in relation to retroviruses and unusual to viruses in general. We therefore propose the following specific aims to better define these retroviral characteristics: 1) to define molecular signals on the HFV glycoprotein which direct budding of HFV virions into intracytoplasmic organelles: 2) to determine whether HFV Gag contains specific signals that target capsids to their intracellular site of budding; 3) to identify the functional significance of transient nuclear transport of the HFV Gag polyprotein; 4) to determine the mechanisms by which HFV Env and Gag interact during virion assembly; and 5) to characterize the mature proteolytic products derived from the HFV Gag polyprotein. HFV offers an excellent opportunity to define new molecular mechanisms which direct virus assembly, protein transport, and virus budding. A better understanding of the human member of this remarkable but relatively neglected genus of retroviruses will enhance our knowledge of both viral and cellular mechanisms.